1. Field of the Invention
The present invention relates to an optical system for light beam scanning, adapted for use in a laser beam printer, a digital copying machine or the like.
2. Related Background Art
Conventional optical systems for such light beam scanning generally utilize light beam deflection by a rotary polygon mirror, as disclosed in the U.S. Pat. No. 4,379,612. If the reflecting faces of such polygon mirror involve an angular error (face inclination) with respect to the rotary axis, the light beam in scanning motion is displaced to undesirably influence the final image output. In order to avoid such undesirable influence of the face inclination, there is already proposed, as described in the above-mentioned patent, to place the rotary polygon mirror and the scanned plane (irradiated plane) in a optically conjugate relationship by means of a toric lens. It is also proposed, as described in the U.S. Pat. No. 4,639,072, to alleviate the influence of the face inclination by positioning a cylindrical lens in the vicinity of the plane scanned by the light beam.
However, in the above-mentioned structure employing a toric lens, in order to correct the aberrations for maintaining desired optical performance, the toric lens has to be close to a plano-convex lens in the shape in the main scanning cross-sectional plane, and a spherical concave lens positioned between the toric lens and the rotary polygon mirror has to be close to a plano-concave or biconcave shape, so that the image angle is inevitably limited. Stated differently, a wider image angle with same optical performance will inevitably require thicker lenses, and as a result, the apparatus will become large.
Also it is generally difficult to manufacture the toric lens, and the use of the toric lens, is a factor of increased cost.
In order to avoid such drawbacks, there can be conceived to prepare the toric lens with a plastic material, but, since said toric lens represents a significant portion in the power of the entire conventional optical system consisting of a concave spherical lens and a convex toric lens from the side of the rotary polygon mirror, the power fluctuation in the plastic lens resulting from changes in ambient conditions is not negligible and results for example in a defocus on the scanned plane.
On the other hand, the other conventional structure utilizing the cylindrical lens in the vicinity of the scanned plane is less affected by the ambient conditions, but the presence of such optical component as the cylindrical lens close to a photosensitive drum is undesirable, for example in an electrophotographic laser beam printer, because various process components such as the developing unit and the cleaning station are positioned close to the photosensitive drum. Also such cylindrical lens, if positioned close to the photosensitive drum, is apt to be subjected to undesirable influence by stain with toner, heat, ozone etc.